my two cents:
Cent 1) Some loads/powders do better if they are packed tightly. I know that my blackhawk is in lust with 15 grains of 2400 under a 158JSP. It will cloverleaf all day, if I could stand to shoot them that long, and would not be surprised if some of that was because it fills the case in a great way. It's not compressed, but if I were to double-charge, it would overflow the case. In a round-about way, seating the bullet deeper in the case would allow less dead-space, or maybe even compress the powder. I personally wouldn't do that, and certainly not with that load, because that load is up there in pressure already.
cent 2) FWIW, I've anecdotally noticed that my accuracy is more dependent on how hard I roll the crimp (which may be related to cent 1, or maybe not). I know rifle rounds don't work that way, and conversely, I know slow powders in the .357 need that hard crimp to let the round build up pressure properly, above needing it to keep the bullet from wandering. I'm not smart enough to break it down without down off the top of my head. Unless you want to ignore the channelature, the COAL is kind of dictated, but the crimp's not.
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